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//===-- Local.cpp - Functions to perform local transformations ------------===//
//
// This family of functions perform various local transformations to the
// program.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/iTerminators.h"
#include "llvm/ConstantHandling.h"
//===----------------------------------------------------------------------===//
// Local constant propogation...
//
// ConstantFoldInstruction - If an instruction references constants, try to fold
// them together...
//
bool doConstantPropogation(BasicBlock::iterator &II) {
Instruction *Inst = *II;
if (Constant *C = ConstantFoldInstruction(Inst)) {
// Replaces all of the uses of a variable with uses of the constant.
Inst->replaceAllUsesWith(C);
// Remove the instruction from the basic block...
delete Inst->getParent()->getInstList().remove(II);
return true;
}
return false;
}
// ConstantFoldTerminator - If a terminator instruction is predicated on a
// constant value, convert it into an unconditional branch to the constant
// destination.
//
bool ConstantFoldTerminator(BasicBlock *BB) {
TerminatorInst *T = BB->getTerminator();
// Branch - See if we are conditional jumping on constant
if (BranchInst *BI = dyn_cast<BranchInst>(T)) {
if (BI->isUnconditional()) return false; // Can't optimize uncond branch
BasicBlock *Dest1 = cast<BasicBlock>(BI->getOperand(0));
BasicBlock *Dest2 = cast<BasicBlock>(BI->getOperand(1));
if (ConstantBool *Cond = dyn_cast<ConstantBool>(BI->getCondition())) {
// Are we branching on constant?
// YES. Change to unconditional branch...
BasicBlock *Destination = Cond->getValue() ? Dest1 : Dest2;
BasicBlock *OldDest = Cond->getValue() ? Dest2 : Dest1;
//cerr << "Function: " << T->getParent()->getParent()
// << "\nRemoving branch from " << T->getParent()
// << "\n\nTo: " << OldDest << endl;
// Let the basic block know that we are letting go of it. Based on this,
// it will adjust it's PHI nodes.
assert(BI->getParent() && "Terminator not inserted in block!");
OldDest->removePredecessor(BI->getParent());
// Set the unconditional destination, and change the insn to be an
// unconditional branch.
BI->setUnconditionalDest(Destination);
return true;
}
#if 0
// FIXME: TODO: This doesn't work if the destination has PHI nodes with
// different incoming values on each branch!
//
else if (Dest2 == Dest1) { // Conditional branch to same location?
// This branch matches something like this:
// br bool %cond, label %Dest, label %Dest
// and changes it into: br label %Dest
// Let the basic block know that we are letting go of one copy of it.
assert(BI->getParent() && "Terminator not inserted in block!");
Dest1->removePredecessor(BI->getParent());
// Change a conditional branch to unconditional.
BI->setUnconditionalDest(Dest1);
return true;
}
#endif
}
return false;
}
//===----------------------------------------------------------------------===//
// Local dead code elimination...
//
bool isInstructionTriviallyDead(Instruction *I) {
return I->use_empty() && !I->hasSideEffects() && !isa<TerminatorInst>(I);
}
// dceInstruction - Inspect the instruction at *BBI and figure out if it's
// [trivially] dead. If so, remove the instruction and update the iterator
// to point to the instruction that immediately succeeded the original
// instruction.
//
bool dceInstruction(BasicBlock::iterator &BBI) {
// Look for un"used" definitions...
Instruction *I = *BBI;
if (isInstructionTriviallyDead(I)) {
delete I->getParent()->getInstList().remove(BBI); // Bye bye
return true;
}
return false;
}
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